Most studies of lipid and lipoprotein metabolism in man have focused on the steady-state conditions present after an overnight fast. Although cholesterol ingestion causes only small increases in the steady-state concentrations of serum cholesterol and LDL, little is known concerning changes in the concentration and composition of high density lipoproteins (HDL) in the period immediately following the ingestion of diets differing in their content of cholesterol. We plan to compare the concentration, composition, size and morphology of various HDL fractions in the sera of normal human subjects between the ages of 20 and 50 during a 4 hour period following the ingestion of a diet containing 100 grams of triglycerides in the presence and absence of 900 mg of added cholesterol. Blood samples will be obtained every 30 minutes. HDLc, HDL2a, HDL2b and HDL3 will be isolated by a combination of heparin precipitation, ultracentrifugation and electrophoresis. Chemical and enzymatic methods will be used to measure free and esterified cholesterol, triglycerides and phospholipids in each lipoprotein fraction. Apoprotein compositions will be determined by polyacrylamide gel electrophoresis and radioimmunoassay. Particle sizes and morphology will be measured by laser beam spectroscopy and electron microscopy. Later studies will examine age and sex-matched patients with types II and IV hyperlipoproteinemias. This study will help to define the short-term changes in HDL when normal and hyperlipidemic subjects are challenged with a cholesterol-rich meal. Transient postprandia changes in the concentration or composition of HDL subfractions may play an important part in the atherosclerotic process.